1. Field of the Invention
This invention relates to instruments which are used particularly, though not exclusively, to place blood embolism filters in the circulatory system of humans or animals.
2. Prior Art
A common cause of death in hospitalized patients is the development of clots in the veins of the lower extremities, also known as deep vein thrombosis. Mortality associated with this condition is due to the occurrence of pulmonary emboli which travel from the lower extremities, through the chambers of the heart, and lodge in the pulmonary artery effectively obstructing blood flow to the lungs.
When a deep vein thrombosis occurs, the initial treatment relies on anti-coagulant therapy, which tends to prevent blood clot propagation as well as pulmonary emboli. However, such treatment is often contraindicated in about a third of all patients, especially in those who have undergone recent major operations due to the increased possibility of bleeding. In these cases, the focus of the treatment shifts to thrombus capture, rather than the prevention of thrombus propagation.
The most widely used method of thrombus capture is placement of a conically-shaped basket filter made of stainless steel into the inferior vena cava. The filter is introduced into the venous system in most instances by way of an incision in the internal jugular vein, and is placed in the appropriate position in the vessel under fluoroscopic control.
Before introduction into the body, the filter is placed with its legs restrained within a stainless steel retainer which is open at one end. The other end of the retainer is attached to a flexible plastic tube, which in turn is secured to a plunger device operated by the surgeon. The instrument is then inserted through the incision made at the jugular vein, and must be maneuvered through an intricate network of blood vessels to reach the lower vena cava destination. When the filter retainer is at the intended position, the filter is pushed out of the retainer by the plunger operated from outside the body. As the filter moves out of the retainer, the legs of the filter expand radially until the filter hooks penetrate the blood vessel wall and cause the filter to attach firmly within the vena cava.
The device currently used to implant blood embolism filters is commonly known as a jugular vein introducer catheter, which has been in use under the commercial name of Greenfield Vena Cava Filter System manufactured by Medi-tech, Inc., and which is described in U.S. Pat. No. 3,952,747 issued to Garman O. Kimmell on Apr. 27, 1976 and entitled "Filter and Filter Insertion Instrument". This device operates in the manner previously described, but has several deficiencies which the invention proposed herein attempts to overcome.
First, the only control that the surgeon has while maneuvering the catheter toward the lower vena cava is by way of the flexion in the plastic tubing connected to the filter retainer. The instrument must negotiate several sharp turns in its passage through the vessels and must do so without injury to the internal vessel walls. In addition, the surgeon must ensure that the instrument does not inadvertently enter any of the other branches leading away from the vena cava. In the current device, such problems may require the surgeon to completely remove the catheter, manually bend the plastic tube to make it conform to the path of the vessels, and reinsert it for a second trial at filter placement. This procedure is necessarily attended with a greater chance of injury to the recipient, as the risks to the patient grow exponentially as the required time for the operation increases. Furthermore, financial cost factors such as operating room time, surgical supplies, and anesthesia are increased significantly as additional time is spent readjusting the instrument during surgery.
Second, the design of the current device allows only for a pushing of the filter into the blood vessel, making it impossible for the surgeon to retract the filter back into the retainer should he decide to place it in a different location. As the filter is pushed by the plunger from the retainer, the position of the filter is irreversible such that the surgeon is helpless to correct the problem by way of the insertion instrument, and the filter may be misplaced or misaligned. Another disadvantage of the current device's inability to retract the filter is due to the inherent tendency of the filter to leave the retainer quickly and erratically during filter expansion as the strain energy in the legs of the stored filter is released. This uncontrollable ejection from the retainer can result in an implantation that is often less then optimum. Consequently, in either of these situations where the filter is improperly placed, the only recourse is to either accept the placement of the filter in its inefficient location, or to remove the filter by major surgery. Both of these alternatives are costly, time-consuming and often dangerous to the patient.
Yet another feature that is lacking in the current device is a means for keeping the filter legs and hooks from entangling each other when the filter is contained within the filter retainer. There is likewise no way of ensuring that the hooks impale the blood vessel equally spaced from one another. The danger of ejecting a filter whose legs are crossed is that the filter may not be implanted securely and may be out of line with the axis of the blood vessel. If this happens, the result could be migration of the filter in the direction of the heart, or in the least, inefficient trapping of emboli in the case of unequally spaced filter legs. The current device may include a filter loader designed to place the filter within the carrier, but there is no method for assuring that the filter legs will remain entangled and equally spaced during the filter's passage to the implantation site.
Finally, the existing art is equipped with a plunger locking device consisting of a threaded knob which tightens against the plunger. While this mechanism serves the purpose of preventing accidental ejection of the filter until the catheter is in the proper location, it is difficult to disengage it with one hand in the operating room environment. Twisting the knob with blood-covered surgical gloves is cumbersome, and does not provide a positive means of engagement or disengagement for the surgeon during the operation and distracts the surgeon's attention from the primary goal of proper filter placement.
Ideally, a filter insertion instrument used to implant blood embolism occlusion devices should not only provide the surgeon with a means for dependably retaining and retracting the filter during the operation, but should also enable the legs to remain equally spaced prior to and during implantation. Such a device should also allow for maximum control of the instrument in its passage through the complex vascular system as well as a locking device which is simple and reliable when used in a surgical environment.